text: Kvitova, 25, will not defend her title at the Sydney International - a warm-up event for the year's first Grand Slam - because of a stomach virus.
  Second seed Radwanska, 26, has also withdrawn because of a leg injury.
  The world's top-four ranked players - Serena Williams, Maria Sharapova, Simona Halep and Garbine Muguruza - have also had injuries this week.
  World number six Kvitova, seeded third for the Sydney tournament, also withdrew from the Shenzhen Open in China last week.
  "I'm really disappointed to have to withdraw from the Sydney International but unfortunately I am still not feeling well," the Czech said.
  Pole Radwanska, the world number five and second seed in Sydney, said she needed to rest her left leg.
  "I'm really sorry that I can't play in Sydney this week," she said. "I am feeling very confident in my game at the moment and the results are there, but after five matches in the past week my leg needs some recovery time."
  Earlier this week, world number one Williams retired with a knee injury when trailing Australia's Jarmila Wolfe in the second set of their Hopman Cup tie in Perth.
  The 34-year-old, who won the Australian Open last year, pulled out of the event before United States' final match of the round-robin tournament.
  Halep, ranked second in the world, withdrew from the Brisbane International with an inflamed Achilles, while world number three Muguruza quit the tournament because of a foot injury.
  World number four Sharapova retired from the same tournament with a forearm injury.
  The Australian Open begins on 18 January.